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Acquisitions & Divestitures - Divestitures (Details) $ in Millions	3 Months Ended				9 Months Ended
	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 divestiture	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Nov. 02, 2022	Nov. 03, 2021
Divestitures
Income/(loss) from discontinued operations, net of tax		$ (10)		$ 18	$ (15)	$ 16
Number of divestitures \| divestiture			2
Kyndryl Holdings, Inc
Divestitures
Income/(loss) from discontinued operations, net of tax		(10)		$ 18	(15)	$ 16
Discontinued Operations, Held-for-Sale \| The Weather Company
Divestitures
Disposal group, assets		531			531
Disposal group, goodwill		450			450
Disposal group, prepaid and other current assets		50			50
Disposal group, intangible assets		20			20
Disposal group, plant, property and equipment		11			11
Disposal group, liabilities		$ 17			$ 17
Discontinued Operations, Held-for-Sale \| The Weather Company \| Forecast
Divestitures
Consideration	$ 1,100
Contingent consideration	250
Cash consideration received	750
Financing receivable	$ 100
Seller financing loan term	7 years
Discontinued Operations, Held-for-Sale \| The Weather Company \| Forecast \| Contingent Consideration, Attainment Of Certain Investment Return Metrics
Divestitures
Contingent consideration	$ 200
Kyndryl Holdings, Inc
Divestitures
Ownership interest by stockholders (in percent)								19.90%
Kyndryl Holdings, Inc \| Kyndryl Holdings, Inc
Divestitures
Kyndryl stock distributed to IBM stockholders								80.10%
Ownership interest by stockholders (in percent)							0.00%